1. Field of the Invention
The present invention relates to an exposure method of photolithography, and particularly to a method of correcting optical proximity effect of contact holes.
2. Description of the Related Art
Photolithography is a process for transferring patterns on a photo mask to a photoresist layer on a wafer. With the rapid increase of the numbers of layers in a semiconductor integrated circuit, the photolithography pattern width is critical. Similarly, because the pitches among semiconductor components are shorter, the size of a transparent region of a photo mask is smaller accordingly. The pitch of a semiconductor component, however, is limited physically by the optical characteristics thereof during an exposure process. Particularly during the development process for forming contact holes, the high-level diffraction produced interfere causes among contact target areas.
This, the optical proximity effect from contact holes is important when design rules are smaller. Although the size of contact holes on a photo mask is constant, the hole patterns densities are different. Contact holes uniformity is thus affected.
One difficulty in correcting the optical proximity effect of contact holes is that the contact holes cannot be corrected individually according to the pattern density. Although some methods can execute correction according to different pitches, the correction of each side of a contact hole may cause the center of a contact hole to shift, producing overlap problems. Moreover, adjacent hole patterns may shift drastically, causing the contact holes to merge after correction.